The Radiological Physics Center (RPC) was established upon the recommendation of the Committee on Radiation Therapy Studies (CRTS), now the Committee on Radiation Oncology Studies (CROS), under the sponsorship of the American Association of Physicists in Medicine (AAPM) to insure the correctness and consistency of radiation dosimetry between institutions involved in interinstitutional clinical trials. Review of dosimetry is accomplished by visits to the institutions by a physicist, irradiation of various mail-out dosimeters, and by review of dosimetry on patients entered into clinical trials. When errors are discovered, the RPC cooperates with the institution to rectify the errors. To date there are 16 active clinical study groups and 2 intergroup efforts served by the RPC. The RPC is presently monitoring 41 active protocols and following patients on 9 closed protocols. In addition there are at least 14 protocols in planning stages. Visits have been made to 178 institutions. Dosimetry discrepancies have been found at 62 institutions of which 51 have been resolved. Efforts are made to develop calculative methods and efficient techniques for measurements that are important to clinical trials. Experience gained in developing quality control procedures for one clinical trial is communicated to other clinical trial groups. This information and other knowledge gained are communicated to the radiological community.